Open Access
Add to BibSonomy
Abstract
Indium-doped zinc oxide (In:ZnO) nanocrystals are successfully produced by a simple refluxed sol-gel technique. The influence of post-heat treatment/ annealing temperatures on the structure, morphology, optical and luminescence properties of nanostructures was investigated using X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), energy dispersion X-ray spectroscope (EDS), UV–Vis and photoluminescence spectroscopies (PL). The XRD results revealed that the synthesized In:ZnO materials are nanocrystalline with a predominant hexagonal wurtzite structure. The average crystallite sizes and lattice constants of the In:ZnO nanoparticles increase with an increase in annealing temperature. SEM micrographs confirmed the nanostructure of the material and showed that the morphologies of In:ZnO nanoparticles varied from prism-like to spindle-like and then to disk-like structures. The reflectance band edge shifted towards longer wavelength while the band gap energy decreased with an increase in annealing temperature. In addition, the PL spectra show a sharp UV and broad yellow-orange emissions in the visible range that shifts slightly due to the influence of annealing temperature. The results illustrate that an optimum property of In:ZnO nanomaterial can be produced when the samples are annealed in the temperature range of 500 to 600 °C.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The zinc oxide nanoparticles (ZnO NPs) are among the most studied inorganic semiconductor materials that have attracted broad attention in the field of nanotechnology for many decades [1,2]. The interest related to the study of ZnO becomes needier because of the development in technologies that allow the nanofabrication of high quality undoped and doped ZnO for variety of electronics and optoelectronics material applications [3,4]. ZnO possess large excitonic binding energy of 60 meV and tunable direct wide band gap of 3.37 eV, which is beneficial for use in high temperature and high power operations and low noise generation, high breakdown voltage functions and near band-edge excitonic emission at room temperature [2,5,6]. It also has unique properties such as good transparency in the visible range of the electromagnetic spectrum, strong luminescence, good conductivity, high electron mobility, excellent thermal stability, high electro- and photo-chemical stabilities etc [7]. Moreover, ZnO has other noticeable features like high surface sensitive catalytic activity in different ambient conditions, abundant in nature, easy to manufacture, non-toxicity, antibacterial, amenability to wet chemical etching and ability to form different nanostructure morphologies [1,7]. These properties make ZnO an interesting and potential material for utilizing in many areas such as transparent conducting oxides (TCO), solar panel displays, antireflective coatings, ultra-violet lasers, light emitting devices, light modulators, photovoltaic cells, transducers, sensor, photocatalysts, piezoelectric devices, flat panel display and others [8,7–12]. ZnO based TCO application has been given a lot of attention due to its high transparency in optical range which is above 90%, high electrical conductivity, less surface roughness, good reproducibility, better etch-ability for different design and pattern, and low growth temperature [9,10]. It is very important to have improved electrical and optical properties of ZnO based TCO for the better energy conversion efficiency of solar cells. This requirement can be achieved by doping intentionally an n-type material with elements like In, Al and Ga in appropriate ratio to enhance the optical, electrical and structural properties of the resulting ZnO nanostructure products. The suitability of these elements is due to their stability and controllability for a specific n-type doping concentration and do have lower vapor pressure compared to others elements [2,13]. Doping with these elements also produces an excellent and modified quality of ZnO nanomaterials for desirable applications listed above. Substitution of indium on the lattice site of ZnO can increase the carrier concentration, mobility, the electrical conductivity and the band gap [9,10] of the crystallite. The photoluminescence emission bands have been observed to blue shift by doping ZnO with indium [2]. Depending on the synthesis condition, the property and quality of the final product of In:ZnO such as the band gap can be changed accordingly. Several physical and chemical synthesis methods have been employed for the fabrication of un-doped and doped ZnO nanomaterials. Deposition methods such as chemical vapour deposition (CVD), pulsed laser deposition (PLD), spray pyrolysis, electrochemical, chemical bath deposition (CBD), precipitation, hydrothermal and sol-gel dip- as well spin-coatings are the most used processing methods [8,11,12,14–18]. Most of these growth methods results into a number of different morphologies and a wide range of material properties of synthesized nanostructures. The draw backs of some of these methods is that they require sophisticated technologies, complicated procedures and are expensive in costs. Sol-gel synthesis method is comparatively simple, inexpensive, reproducible and reliable technique to produce ZnO nanomaterials with excellent quality, different surface morphological structures and improved properties [11]. This solution method is not only selected for ZnO rather appropriate for the production of any undoped and doped metal oxide compounds [19,20]. Besides selecting an effective method and appropriate doping element, varying and controlling the synthesis parameters during production are other important variables in order to produce an improved quality of doped ZnO nanomaterials. One of these influential parameters affecting the product to be controlled is the post heat treatment condition such as annealing temperature, annealing period and annealing atmosphere [8,21]. It’s known that the annealing processes are used to improve the crystal quality, reduce defects and also to confirm the stability of the crystal at a different temperature, which is important for device performance purposes [3]. Therefore, annealing temperature has significant influence on the structural, morphological, optical and electrical properties of In:ZnO. Annealing at lower temperature may results into poor crystallized In:ZnO structure that directly affects the optical and electrical properties of the final product while annealing at high temperature may also lead to poor material property that causes breakages in bonds between atoms and create defects or some other unexpected compounds. Most of the previous or earlier reports primarily focus exclusively on the annealed properties of un-doped ZnO. It is therefore of interest to consider the annealing effects on doped ZnO in order to produce In:ZnO nanostructures with an excellent properties for appropriate application. To the best of our knowledge, there are few reports on the influence of annealing temperature on the properties of In:ZnO.
Thus, the aim of the present work was the production of In:ZnO nanostructures with an improved material properties using sol-gel method at variable annealing temperature. In order to enhance the possible potential of up scaling of the sol-gel method, the focus was set on the easiest and cheapest possible aqueous-based solution recipes, as well as controlled refluxed synthesis technique. Several characterization techniques were applied to study the dependence of the properties of the produced In:ZnO nanostructures on the annealing temperature.
2. Experiments and measurements
2.1. Materials and procedures
In-ZnO NPs were fabricated by simple refluxed sol-gel method. A 0.18 M of zinc acetate dihydrate [Zn(CH3COO)2·2H2O; 99.59%], 1 M of sodium hydroxide [NaOH; 99.5%], 4.5 at.% of indium nitrate hydrate [In(NO3)3·xH2O; 99.99%] and 1 ml of monoethanolamine [MEA] which were used as precursors without further purification were dissolved in 100 ml deionized water solvent. The detailed synthesis processes for the production of In:ZnO nanostructures is presented in our previous report [22]. The synthesis temperature and pH of the solution was maintained constant at 75 °C and 9, respectively. The solution was refluxed in an oil bath to maintain the temperature and volume of the solution constant and the solution initially changed to a thick milky gel-like when stirred constantly with magnetic stirrer for 2 h.
The general chemical reaction process for the formation of In:ZnO in aqueous synthesis condition can be proposed to be the following [23];
The resulting precipitate are stored for 24 h at room temperature and later filtered. Then the In-ZnO nanoparticle were dried at 75 °C for for 1 h to remove any volatile components. The resultant samples are mixed and grounded to make them uniform. Finally, six samples are annealed at different annealing temperature of 300, 400, 500, 600, 700 and 800 °C for 2 h in air and collected for further characterization.
2.2. Characterization techniques
Several characterizations were applied to acquire different properties of the produced In:ZnO nanostructures at variable annealing temperatures. Bruker D8 Advance X-ray diffractometer at incident radiation 1.5406 Å wavelength from CuKα source were used to compute the crystallographic structure phase, and related constants of the crystallite. By comparing with the Joint Committee on Powder Diffraction Standards (JCPDS) the obtained values were analyzed. Jeol JSM-7800F field emission electron microscope (FE-SEM) were applied to acquired the surface morphology of the produced In:ZnO nanostructures. The stoichiometry of the constituent elemental composition of the prepared nanocrystals were obtained from Oxford energy dispersive X-ray spectroscopy (EDS) using Aztec software. Additional morphological and structural confirmations were acquired from Philips CM 100 transmission electron microscope (TEM). Studies of the optical property and energy band gap of the resulting In:ZnO nanoparticles were done using perkinElmer Lambda 950 ultraviolet-visible (UV-Vis) spectrometer. Photoluminescence (PL) measurements were carried out at room temperature to study the optical luminescence properties and crystallinity of the produced nanocrystals. A He-Cd laser with wavelength of 325 nm was used from Kimmon IK Series laser system for PL measurements.
3. Results and discussions
3.1. Structural analysis
Figure 1 shows the XRD patterns of the produced indium doped ZnO nanostructures at different annealing temperature from 300 to 800 °C for the same time duration. All the expected characteristic diffraction peaks of ZnO between 30° and 70° were observed. It reveals that all nanoparticles are polycrystalline hexagonal wurtzite ZnO in structure [13,24]. The pattern matches well with space group P63mc of JCPDS diffraction card file No. 36-1451.
Fig. 1. XRD spectra of refluxed sol-gel synthesized In:ZnO nanostructure at various annealing temperatures.
At higher annealing temperature exceeding 700 °C some impurity peaks emerges with lower intensities at a diffraction 2θ angles of 30.4168, 35.1997, 51.0237 and 60.5134° (indicated by asterisk in Fig. 1) which can be assigned to the planes (222), (400), (440) and (622) of indium oxide (In2O3), respectively [13,14,25]. The impurity phase formation at higher annealing temperature may be due to In atoms getting higher thermal energy and diffuse out of the lattice sites towards the grain boundary. Furthermore, high annealing temperature can cause cracks and porosity (confirmed in SEM images) in ZnO bond structure. The bonds break providing the possibility of oxygen and indium atoms to migrate and form indium-oxygen matrix (In2O3) between the interstitial unbonded atoms [26]. The observed broad and intense diffraction peaks indicate the formation of highly crystalline nanostructures. Relative comparison of the intensities of all the peaks of the spectra are seen to strengthened and increase linearly with an increase in annealing temperature from 300 to 800 °C, confirming the improvements in crystalinity of the produced In:ZnO nanocrystals [27]. Furthermore, a slight shift in diffraction peaks to lower angles was observed when compared to the reference due to the influence of substitution of larger sized In3+ ions at the Zn2+ ion lattice site. It is evident in the spectra that the diffraction peaks shifted more to the lower angle as the annealing temperature increased, signifying that more indium ion is substituted in the lattice sites as temperature increased. This is due to the fact that as the temperature increased during annealing process the constituent atoms in the compound attain higher thermal energy to rearrange themselves in the lattice sites [2]. Due to this facilitated condition the incorporation of indium ion to the sites of zinc ion can be increased and the diffraction peaks shift more towards the lower angle. A little inconsistency of peak shift towards higher angle at lower annealing temperature may be because of some dopant atoms are not substituted the lattice sites rather they are interstitial atoms. At lower thermal energy atoms may not get enough energy to substitute the host matrix, but they can be at interstitial which create additional stress on the structure. In addition, the decrease of peak broadening is nearly linearly as annealing temperature increase, Fig. 2(a), indicate the enhancement of crystallinity and grain growth due to coalescences [2,28]. At annealing temperatures, ranging up to 600 °C the studied samples does not include any foreign phases. However, the presence of an In2O3 impurity at temperature above 700 °C reveals an important information that using very high annealing temperatures cause the formation of extra unexpected phases in the compound. Accordingly, one can infer that the choice of an appropriate annealing temperature found to be between 500 and 600 °C in this study can improve the quality of the In:ZnO nano crystal. The inter-planar spacing (d) for hexagonal crystal system can be related to the lattice parameters using Bragg’s law λ = 2dhkl Sin(θ) for first order diffraction and the equation [29]:
Fig. 2. Relation between annealing temperature with (a) crystallite size & peak broadening and (b) the lattice constants a & c of the produced In:ZnO nanocrystals.
The crystallite sizes (D) of the produced samples was evaluated from the broadening of the diffraction spectrum peaks using the Scherrer formula [8]
where K is a constant value taken as 0.9, λ is the incident X-rays source wavelength and β is the FWHM in rad of the Bragg’s diffraction peak at an angle θβ. The numerical value for the crystallite sizes of the nanocrystals increased from 25.5 to 31.6 ± 0.3 nm as the annealing temperature increased from 300 to 800 °C, respectively. The gradual crystal growth may be because of the fast movement of constituent atoms caused by the increased thermal energy to recrystallize themselves with adjacent favourable particle and also the coalescence of smaller particles by diffusion in the grain boundary to form larger grain [2,28]. The mechanism of the increased crystallite size include transfer of atoms at the grain boundary by diffusion from one grain to another depending on their energy gained from the annealing temperature. In addition to that the incorporation of larger size In3+ ion in the place of host site may create additional tensile stain to increase the crystal size [22,14]. The observation of the relation between crystallite size and annealing temperature is exhibited in Fig. 2(a) as well in Table 1. The volume of the crystallites (Vcryst) can be computed from the results of crystallite sizes by the relation ${V_{cryst}} = {D^3}$ [43]. The obtained average value of the volumes of all In:ZnO nanocrystals is 16.2 × 103 nm3. The number of unit cells in the single crystallite can be obtained from the ratios of volume of a crystallite to volume of unit cell (${N_{unit}} = {V_{cryst}}/{V_{unit}}$). As it was expected, the number of unit cells increased with annealing temperatures. The obtained results are presented in Table 1. In comparison, the number of unit cells in the crystallite for annealing temperature 800 °C is found to be 4.8 × 105 in which is approximately 2.3 times larger than the value obtained for annealing temperature of 300 °C.
Table 1. Crystallographic structural parameters determined from XRD measurements at different annealing temperatures.a
The density of dislocations (δ) per unit area in the produced crystallite was estimated using the relation $\delta = \frac{1}{{{D^2}}}$ where D is crystallite size obtained from Scherrer formula. The average value for the dislocation density was found to be 1.235 × 1011 cm-2. The overall dislocation density for the fabricated In-ZnO decreased nearly linearly following the values of the crystallite sizes (Table 1). The decreased in the density of dislocation was expected due to the fact that increasing the annealing temperature dissociates some defects in the crystal structure.
Moreover, the crystal imperfections and distortions or micro-structural strain (ε) in the nanocrystals has been examined using the Williamsons-Hall formula from the peak broadenings (β) [32,15].
A graph of $\beta cos(\theta )$ versus $4sin(\theta )$ was plotted as in Fig. 3 to estimate the strain and crystallite sizes from the slope and intercepts, respectively. It is observed that the structural strain decreased as the annealing temperature increased until 600 °C and decreased thereafter (Table 1). The decrease in strain can be attributed to the disappearance of some intrinsic defects like interstitial and vacant atoms in the crystal due to the increase in annealing temperature [21]. The formation of In2O3 crystal phase after 700 °C affects the structure by inducing extra tensile strain on the wurtzite nanocrystal construction. It is found that the determined crystallite size values obtained from Williamson-Hall’s equation seem to increase following similar trend as the values obtained by using Scherrer equation. The determined crystallite sizes increased from 37.5 to 47.4 ± 0.4 nm with annealing temperature. Comparison of the obtained values of crystallite sizes from Scherrer and Williamson-Hall has been done and presented in Table 1. A slightly greater value of the crystallite sizes for Williamson-Hall than Scherrer equation was expected theoretically due to the positive strain developed in all samples [43]. It is also observed that as $4sin(\theta )$ is setted to be 0, the value of $\beta cos(\theta )$ be equal to the value of broadening that was obtained using scherrer formula without the effect of microstructural strain. This result can verify that the broadening of the XRD spectra is affected by both the crystallite size and microstructural strain.
3.2. Composition and morphology analysis
Figure 4 displays the EDS spectra of the synthesized In:ZnO nanomaterials at variable annealing temperatures. The spectra disclose that the obtained nanopowders were composed of only indium (In), zinc (Zn) and oxygen (O) in considerable ratio as expected. The result confirms the XRD measurements that there is no presence of extra impurity elements included in the compound. The small trace of element carbon is attributed to the tape used for sample characterizations.
Fig. 4. EDS spectra for the produced In:ZnO nanostructure at; (S1) 300, (S3) 500, (S4) 600, and (S6) 800 °C annealing temperatures.
The obtained weight percent is converted to atomic percent for better understanding of the compound stoichiometric ratio. The amount of dopant indium and host zinc (In, Zn) ions in the compound are estimated to be (3.09, 48.00), (3.04, 49.82), (3.33, 48.89), (3.31, 47.53), (2.88, 46.62) and (2.84, 48.97) at.% for samples annealed at temperatures of 300, 400, 500, 600, 700 and 800 °C, respectively. A slight reduction of In content in In-ZnO nanoparticles is noted as the annealing temperature increased may due to evaporations of volatile indium oxide. The atomic percentage of each element in the compound (In:ZnO) was nearly similar as expected for all samples that are annealed at different temperature.
The surface morphologies of In-doped ZnO samples prepared at different annealing temperature are exhibited by the FE-SEM micrograph in Fig. 5. The produced crystal structures have a mixture of solid hexagonal prism- and spindle-like morphology. It is known that the formation of prism and spindle like structures of the In:ZnO crystals can be promoted by using ethanolamine compounds like monoethanolamine (MEA) [24,16]. At lower annealing temperature 300 °C presented in Fig. 5(a), the surface morphology displays aggregated hexagonal prisms structures aligned irregularly. Each prism-like structure was made from two connecting prisms of almost similar in size. A slight offset between the connecting faces and edges of the crystal is observed. In addition, it is noted in some of the prisms fractures, nodules and breakages occur at the tips as well on the surfaces due to the use of MEA base as previously reported [24]. Previous reports show that such hierarchical structures formed on the tips of the prisms can improve the light harvesting property of TCO for solar cell applications [33]. The average size of the prism was 900 nm in length and 254 nm in width at the middle of the structure with aspect ratio of 3.54.
Fig. 5. FE-SEM images of the fabricated In:ZnO nanostructures at different annealing temperatures of (a) 300, (b) 400, (c) 500, (d) 600, (e) 700 and (f) 800 °C.
The general proposed mechanism of the evolution of 3D nanostructures involves the Ostwald-ripening process [34]. The originally formed nuclei hexagonal nanoplates/disks grows in size through adsorption on the surface and got stacked together and led to the formation of nano-prisms and spindles. The layer-by-layer stacking of hexagonal disks with each other is expected to be occurred on both sides of the first formed nanodisk nuclei to form two connecting prisms [17]. Increasing the annealing temperature to 400 °C increases the aggregation of prism and spindle like structures; primarily the structures are spindle-like as it seen in Fig. 5(b). Some fluffy structures are noticed on the surfaces of the structures, may be due to some adsorbed In3+ ions on the surface of the structures. The mean size of the spindles is changed to 665 nm in length and 282 nm in width. The produced sample structures at 500 °C annealing temperature in Fig. 5(c) reveals a smaller size nanoprisms with cracks and broken surfaces caused by the increased annealing temperature. At this stage, the structures are densely compact due to the higher temperature and the sizes are reduced to 649 nm in length and 213 nm in width with aspect ratio of 3.05. Mostly spindle like morphological structures are obtained when annealing temperature was changed to 600 °C. It is observed in Fig. 5(d) that smaller and bigger spindles are mixed and randomly arranged with some fluffs distributed on the surfaces. The smoothness of the surfaces for spindles is improved at this stage. The sizes of the spindles at this stage increased to 760 nm in length and 280 nm in width with aspect ratio of 2.72. Further increase in the annealing temperature modifies the morphology to smaller hexagonal compact nanodisks without further growth into prism and spindles (Fig. 5(e) and (f)). This is occurred at higher annealing temperature above 600 °C in which the In3+ ions start forming cubic phase In2O3 compounds with the nearly neighbor free oxygen ions. The competition of In3+ and Zn2+ ions to form their phase structure prevents the growth speed of prism and/ or spindle structures, short and denser hexagonal disks are stacked together to form very short hexagonal prisms. The average widths of these disk-like structures are measured to be 290 and 270 nm for the sample annealed at 700 and 800 °C, respectively. Very small nanoparticles and tiny needle fluff structures are distributed over the surface of the disk-like structures. These structures are expected to be from the formation of indium oxide compound as the presence of impurity is confirmed in XRD measurements.
A closer look at the produced nanostructures was performed by transmission electron microscopy (TEM) to get more significant details. As it is exhibited in Fig. 6(a) to (d), the nanostructures are approximately prism, spindles and disk-like in shape, confirming the findings obtained by SEM measurements. The smaller fluffy and nodule particles from the breakages are clearly seen around the surfaces of the prisms and spindles structures. The lengths and widths of of the nano-prisms, spindles and disks approximately approaches the values obtained from SEM results. The estimated average sizes of the prisms or spindles changed from 852 to 769 nm in length and 320 to 250 in width. Similarly the width of the disk-like structure for the sample annealed at 800 °C is estimated to be around 60 nm.
Fig. 6. TEM micrograph of the produced In:ZnO nanomaterials at various annealing temperatures (a) 400, (b) 500, (c) 600 and (d) 800 °C.
3.3. Optical and photoluminescence studies
3.3.1. UV-Vis measurement studies
The reflectance (R) spectrum of all the produced In:ZnO nanocrystals at different annealing temperature in the wavelength range of 200 - 800 nm are presented in Fig. 7(a), and the corresponding absorbance (α) spectra of the illustration are determined using the Kubelka-Munk function [15];
where R is the reflectance and fKM is proportional to the absorbance. The absorption band edge was red shifted as annealing temperature increased from 300 to 800 °C associated with the formation of larger crystallite sizes with strong excitonic absorption and due to debilitated quantum confinement effect [21,35]. There is no significant change of the reflectance spectrum of the samples in the lower UV range. It is observed that the band edge shifted to longer wavelength with increase in annealing temperatures; implying the corresponding decrease of the band gap energy of the nanocrystals. The percentage reflectance in the visible region of the spectrum is appreciably higher than 90%. In comparison, the reflectance in the visible region increased as annealing temperature increased, and the maximum reflectance is observed at 800 °C. The increase in reflectance in the longer-wavelength range with annealing temperature primarily attributed to the decreased absorption of light by the nanostructure, which hardly correlated to any contribution to change the luminescence properties.Fig. 7. (a) Absorbance with reflectance (inset) spectra and; (b) Tauc plots for band gap estimation for the produced In:ZnO nanocrystals at variable annealing temperatures.
To find out the energy band gap of the produced nanocrystals, the Tauc’s relation in which the absorption coefficient (α) is correlated to the incidented photon energy (hν) and band gap (Eg) in eV as [36]
where C is material constant.The energy band gaps of the In:ZnO samples are determined from the graph of (αhν)2 versus hν by extrapolating the linear portion of the plot to the horizontal hν axis. Figure 7(b) depicts the plots relation of the band gap for the fabricated nanostructure samples annealed at various temperatures. The obtained energy band gap is found to be temperature dependent as disclosed in Fig. 8, where the band gap is observed to decreases nearly linearly as the annealing temperature increases. The band gap energy varies between 3.24 and 3.19 ± 0.005 eV as annealing temperature changes between 300 to 800 °C (Table 2). The obtained results are in the optimum applicable range of In:ZnO band gap energies from 3.0 to 3.4 eV for optoelectronics applications such as in TCO, LED and solar cells [8,9] when thin films can produced. The observed decrement in band gap energy is expected to be due to the expanded crystallite sizes caused by the increased annealing temperature and the effect of tensile strain induced by doping larger size In3+ ions in the host sites [37,38]. The incorporated dopant atoms create strain by occupying the grain boundaries or interstitial sites. The existence of dopant impurity atoms in the crystals induce the narrowing of band gaps due to the formation of new recombination states below the band gap with lower emission energies [30,21]. Moreover, the increased annealing temperature may cause to shrink the band gap with the incorporation of dopant atom specifically the Fermi level [39,40]. The influence of an emerged impurity of cuboid indium oxide phase on the band gap is also observed after annealing temperature of 700 °C.
Fig. 8. Graphical analysis of the relation between energy band gap of In:ZnO nanostructured samples with annealing temperature.
Table 2. Optical property parameters of In:ZnO nanostructures various annealing temperatures.
A model empirical relation by Herve and Vandamme [31,41] is used to estimate the refractive index (n) of the produced nanocrystals;
where A = 13.6 eV and B = 3.4 eV are constants.The calculated mean value of the refractive indices was found to be 2.286, which is consistent with the report [31]. The variations of the values depend on the obtained energy band gap, in which the refractive index increased as energy band gap decreased. The optical dielectric constant ${\varepsilon _\infty }$ can also be estimated from the relation ${\varepsilon _\infty } = {n^2}$ using the obtained values of refractive indices [31]. The calculated average value of the dielectric constants was 5.226, the change follows the changes in the refractive indices. Table 2 presents the obtained values of band gap, refractive indices and dielectric constants in relation to the variation of annealing temperature.
3.3.2. Photoluminescence studies
Photoluminescence (PL) examinations are an efficient method to investigate the defects of the samples nanomaterials. Figure 9 displays the room temperature PL spectra of simple sol-gel synthesized In:ZnO nanocrystals at different annealing temperatures between 300 and 800 °C. The spectra of the all measured samples exhibit two obvious emission bands, one in the UV region corresponds to the near band emission (NBE) which originate from free excitonic transitions, and the other band corresponds to deep level emission (DLE) which arises from defect level transitions like zinc and/or oxygen vacancies, interstitials and antisites [15,21,29,42].
Fig. 9. Room temperature photoluminescence spectra for In:ZnO nanocrystallites at variable annealing temperatures.
The intensity of the narrow band excitonic emissions of the crystals seem to be suppressed as annealing temperature increased, implying a decline in crystal quality due to the formation of defect band levels in the band gap to trap the transition of photon carriers or excitons during recombination process [15]. The incorporation of larger size dopant in the lattice sites and interstitial create structural strain and induce extra defects in the band gap, which can be a source of DLE band emissions with higher intensity than NBE band emissions. The FWHM of the excitonic band peak seen clearly to decrease as annealing temperature increases, indicating the improvements in crystallinity of the prepared In:ZnO nanocrystllites [18], which was also confirmed by the XRD results. The characteristic NBE band in the UV region corresponds mainly to the free exciton (Fx) recombination’s. It is also observed in the spectra that the excitonic band center is shifted towards shorter wavelength as increased annealing temperature. Figure 10 presents the relation of NBE band shift with annealing temperature, in which the wavelength of the emitted photon radiation shifted from 388 to 382 ± 0.7 nm as annealing temperature increased from 300 to 800 °C. The blue shift can be attributed to the enhancements in quality of the produced nanomaterials caused by the increased annealing temperature. The emission at the UV region has a potential application in the area of ZnO based UV LEDs, photon detectors and UV sensors [29].
Fig. 10. Analysis of PL NBE peak position shift and intensity ratio (INBE/IDLE) of the emission peaks of the nanostructures produced at different annealing temperatures.
The DLE bands in the visible region arising from the intrinsic and extrinsic defects are seen to deviate to longer wavelength until the annealing temperature becomes 700 °C. The possible source of this emission band is expected to be the oxygen vacancy and/or zinc interstitials [2,21,29,42]. In addition to that the incorporated indium ion in the crystal structure also have its own contribution in this emission band [43,44]. The shift of the visible emission band can be explained by the various chemical surroundings of the defect emission levels like In3+ ions and the formation of the compound In2O3 in the sample. In addition to that the increased density of defects and their interaction with each other also can cause shift of the visible emission band [15]. The observed emission band change to 570 nm for the sample annealed at 800 °C due to oxygen vacancy in the cuboid indium oxide phase that emerged at higher temperature [25,13]. It is confirmed by the XRD measurements that annealing at higher temperature causes for the emergence of cuboid indium oxide phases, which can be one of the source for the shift in visible emission band. The formation of such a cuboid phase in the sample also favors the creation of additional oxygen vacancies in the crystal to have an emission in orange-red region of the spectrum [25]. Therefore, annealing at higher temperatures above 600 °C initiates the formation of excess density of indium oxide phase defects with the shift of visible emission band. The intensity of the visible band (IDLE) is seen to decrease as the annealing temperature increased. Comparison of the quality of the produced ZnO nanostructures was done by examining the ratios of the UV and visible emission intensities (INBE/IDLE). In Fig. 10 it is clearly seen that the PL intensity ratio of the produced samples decreases slightly as annealing temperature increased, indicating the increased structural defects induced by the incorporated of larger size indium dopant in the crystallites. The visible defect level luminescence property of the In:ZnO nanomaterial may be useful in the area of optoelectronics and biomedical device applications [2,45].
4. Conclusions
In:ZnO nanostructures were successfully synthesized at lower growth temperature (75 °C) using simple sol-gel method. The dependence of the material properties of In:ZnO on the annealing temperature is investigated in the temperature range 300 - 800°C. XRD analysis verify an improved crystallinity with the growth in crystallite sizes from 25.5 to 31.6 nm as annealing temperature increased from 300 to 800 °C due to the incorporated larger size dopant and the obtained higher thermal energy for recrystallization. Prism-, spindle- and disk-like morphology of the produced crystals were confirmed with the help of scanning and transmission electron microscopy. The stoichiometry of the constituent elements traced by EDS measurement discloses an appropriate ratio without any impurity when annealed at moderate temperatures. The shrinkage in optical energy band gaps from 3.24 to 3.19 was observed because of the expanded crystallite size. Radiative band-to-band recombination of excitons and defect level transitions of electrons in the band gap produce emission bands at UV (∼382 nm) and visible, yellow-orange, regions of the spectrum. Both emission band intensities were quenched as annealing temperature increased. The spectral positions of the bands are found to be annealing temperature dependent, where the bands are blue shifted in the whole annealing temperature range. The observed shift of the radiative NBE band could be due to the enhancements in quality of the crystallite nanomaterial as a result of the increased annealing temperature. The results show that an optimized In:ZnO nanomaterial can be produced when samples are annealed in the temperature range of 500 to 600 °C. These findings indicate that In:ZnO nanostructures with improved properties can be synthesized which may lead to possibility of several promising potential application in the area of electronics, biomedical and optoelectronics.
Funding
National Research Foundation; Dire Dawa University; Ministry of Science and Higher Education.
Acknowledgments
Authors are grateful to the financial support from the Ministry of Science and Higher Education (MoSHE) and Dire Dawa University, Ethiopia, and National Research Foundation (NRF), through the directorate of research of the University of the Free State, South Africa. We also need to thank Mr Edward Lee for his assistance in center of microscopy lab for SEM and TEM measurements for this study.
Disclosures
The authors declare no conflicts of interest.
References
1. D. Daksh and Y. K. Agrawal, “Rare Earth-Doped Zinc Oxide Nanostructures: A Review,” Rev. Nanosci. Nanotechnol. 5(1), 1–27 (2016). [CrossRef]
2. H. Morkoç and Ü. Özgür, (2009), Zinc Oxide Fundamentals, Materials and Device Technology (Wiley, 2009).
3. S. S. M. Ismail and A. R. Ainuddin, “Effect of Annealing Temperature on the Properties of Transition Metal Doped Zinc Oxide – A Review,” AIP Conf. Proc. 2068, 020101 (2019). [CrossRef]
4. Ü. Özgür, M. Hofstetter, and H. D. Morkoç, “ZnO Devices and Applications: A Review of Current Status and Future Prospects,” Proc. IEEE 98(7), 1255–1268 (2010). [CrossRef]
5. C. F. Klingshirn, B. K. Meyer, A. Waag, A. Hoffmann, and J. Geurts, “Zinc Oxide From Fundamental Properties Towards Novel Applications, Springer series in materials science,” Springer-Verlag Berlin Heidelberg (2010).
6. C. W. Litton, D. C. Reynolds, and T. C. Collins, (2011), Zinc Oxide Materials for Electronic and Optoelectronic Device Applications (Wiley, 2011).
7. A. B. Djurišić, A.M.C. Ng, and X. Y. Chen, “ZnO nanostructures for optoelectronics: Material properties and device applications,” Prog. Quantum Electron. 34(4), 191–259 (2010). [CrossRef]
8. S. Benramache and B. Benhaoua, “Influence of annealing temperature on structural and optical properties of ZnO: In thin films prepared by ultrasonic spray technique,” Superlattices Microstruct. 52(6), 1062–1070 (2012). [CrossRef]
9. K. Ellmer, A. Klein, and B. Rech, (2007), “Transparent Conductive Zinc Oxide Basics and Applications in Thin Film Solar Cells,” (vol 104), ©2008 Springer-Verlag Berlin Heidelberg, ISBN 978-3-540-73611-0 Springer, Berlin Heidelberg New York.
10. Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, and H. Morkoç, “A comprehensive review of ZnO materials and devices,” J. Appl. Phys. 98(4), 041301 (2005). [CrossRef]
11. L. Znaidi, “Sol–gel-deposited ZnO thin films: A review,” Mater. Sci. Eng., B 174(1-3), 18–30 (2010). [CrossRef]
12. K. Agnieszka and J. Teofil, “Zinc Oxide-From Synthesis to Application: A Review,” Materials 7(4), 2833–2881 (2014). [CrossRef]
13. J. Wang, F. Cheng, Y. Liang, H. Chen, C. Tsai, C. Fang, and T. Nee, “Anomalous luminescence phenomena of indium-doped ZnO nanostructures grown,” Nanoscale Res. Lett. 7, 270 (2012). [CrossRef]
14. K. S. Khashan and M. Mahdi, “Preparation of indium-doped zinc oxide nanoparticles by pulsed laser ablation in liquid technique and their characterization,” Appl. Nanosci. 7(8), 589–596 (2017). [CrossRef]
15. T. K. Pathak, A. Kumar, C. W. Swart, H. C. Swart, and R. E. Kroon, “Effect of fuel content on luminescence and antibacterial properties of zinc oxide nanocrystalline powders synthesized by the combustion method,” RSC Adv. 6(100), 97770–97782 (2016). [CrossRef]
16. J. M. Hancock, W. M. Rankin, B. Woolsey, R. S. Turley, R. G. Harrison, and J. Sol-Gel, “Controlled formation of ZnO hexagonal prisms using ethanolamines and water,” J. Sol-Gel Sci. Technol. 84(1), 214–221 (2017). [CrossRef]
17. N. Qin, Q. Xiang, H. Zhao, J. Zhang, and J. Xu, “Evolution of ZnO microstructures from hexagonal disk to prismoid, prism and pyramid and their crystal facet-dependent gas sensing properties,” CrystEngComm 16(30), 7062 (2014). [CrossRef]
18. Z. N. Urgessa, O. S. Oluwafemi, J. K. Dangbegnon, and J. R. Botha, “Photoluminescence study of aligned ZnO nanorods grown using chemical bath deposition,” Phys. B 407(10), 1546–1549 (2012). [CrossRef]
19. F. Xie, W. C. H. Choy, C. Wang, X. Li, S. Zhang, and J. Hou, “Low-Temperature Solution-Processed Hydrogen Molybdenum and Vanadium Bronzes for an Efficient Hole-Transport Layer in Organic Electronics,” Adv. Mater. 25(14), 2051–2055 (2013). [CrossRef]
20. Z. Huang, D. Ouyang, R. Ma, V. A. L. Roy, W. Wu, and W. C. H. Choy, “A General Method: Designing a Hypocrystalline Hydroxide Intermediate to Achieve Ultrasmall and Well-Dispersed Ternary Metal Oxide for Efficient Photovoltaic Devices,” Adv. Funct. Mater. 29(45), 1904684 (2019). [CrossRef]
21. R. A. Wahyuono, F. Hermann-Westendorf, A. Dellith, C. Schmidt, J. Dellith, J. Plentz, M. Schulz, M. Presselt, M. Seyring, M. Rettenmeyer, and B. Dietzek, “Effect of annealing on the sub-bandgap, defects and trapping states of ZnO nanostructures,” Chem. Phys. 483–484, 112–121 (2017). [CrossRef]
22. E. T. Seid and F. B. Dejene, “Controlled synthesis of In-doped ZnO: the effect of indium doping concentration,” J. Mater. Sci.: Mater. Electron. 30(12), 11833–11842 (2019). [CrossRef]
23. J. H. Lim, S. M. Lee, H. Kim, H. Y. Kim, J. Park, S. Jung, G. C. Park, J. Kim, and J. Joo, “Synergistic effect of Indium and Gallium co-doping on growth behavior and physical properties of hydrothermally grown ZnO nanorods,” Sci. Rep. 7(1), 41992 (2017). [CrossRef]
24. J. M. Hancock, W. M. Rankin, B. Woolsey, R. S. Turley, and R. G. Harrison, “Controlled formation of ZnO hexagonal prisms using ethanolamines and water,” J. Sol-Gel Sci. Technol. 84(1), 214–221 (2017). [CrossRef]
25. G. Liu, “Synthesis, Characterization of In2O3 Nanocrystals and Their Photoluminescence Property,” Int. J. Electrochem. Sci. 6, 2162–2170 (2011).
26. P. K. Ooi, S. S. Ng, M. J. Abdullah, and Z. Hassan, “Structural and optical properties of In-doped ZnO thin films under wet annealing,” Mater. Lett. 116, 396–398 (2014). [CrossRef]
27. P. Gu, X. Zhu, and D. Yang, “Effect of annealing temperature on the performance of photoconductive ultraviolet detectors based on ZnO thin films,” Appl. Phys. A 125(1), 50 (2019). [CrossRef]
28. J. Sengupta, R. K. Sahoo, and C. D. Mukherjee, “Effect of annealing on the structural, topographical and optical properties of sol–gel derived ZnO and AZO thin films,” Mater. Lett. 83, 84–87 (2012). [CrossRef]
29. V. Kumar, V. Kumar, S. Som, A. Yousif, N. Singh, O. M. Ntwaeaborwa, A. Kapoor, and H. C. Swart, “Effect of annealing on the structural, morphological and photoluminescence properties of ZnO thin films prepared by spin coating,” J. Colloid Interface Sci. 428, 8–15 (2014). [CrossRef]
30. J. P. Mathew, G. Varghese, and J. Mathew, “Effect of Annealing on the Optical Properties of Transition Metal Doped ZnO Thin Films IOP Conf,” IOP Conf. Ser.: Mater. Sci. Eng. 73, 012065 (2015). [CrossRef]
31. K. L. Foo, U. Hashim, K. Muhammad, and C. H. Voon, “Sol-gel synthesized zinc oxide nanorods and their structural and optical investigation for optoelectronic application,” Nanoscale Res. Lett. 9, 429 (2014). [CrossRef]
32. M. K. Hussen, F. B. Dejene, and M. Tsega, “Effect of pH on material properties of ZnAl2O4:Cr3+ nano particles prepared by sol–gel method,” J. Mater. Sci.: Mater. Electron. 30(11), 1355 (2019). [CrossRef]
33. R. Vittal and K. Ho, “Zinc oxide based dye-sensitized solar cells: A review,” Renewable Sustainable Energy Rev. 70, 920–935 (2017). [CrossRef]
34. S. K. Arya, S. Saha, J. E. Ramirez-Vick, V. Gupta, S. Bhansali, and S. P. Singh, “Recent advances in ZnO nanostructures and thin films for biosensor applications: Review,” Anal. Chim. Acta 737, 1–21 (2012). [CrossRef]
35. S. S. Alias, A. B. Ismail, and A. A. Mohamad, “Effect of pH on ZnO nanoparticle properties synthesized by sol-gel centrifugation,” J. Alloys Compd. 499, 231–237 (2010). [CrossRef]
36. A. Wang, T. Chen, S. Lu, Z. Wu, Y. Li, H. Chen, and Y. Wang, “Effects of doping and annealing on properties of ZnO films grown by atomic layer deposition,” Nanoscale Res. Lett. 10(1), 75 (2015). [CrossRef]
37. J. P. McKelvey and Solid State, Semiconductor Physics (Robert E.Krieger Publishing Company Malabar, 1966), p. 257
38. A. Hafdallah, F. Yanineb, M. S. Aida, and N. Attaf, “In doped ZnO thin films,” J. Alloys Compd. 509(26), 7267–7270 (2011). [CrossRef]
39. X. Li, F. Xie, S. Zhang, J. Hou, and W. C. H. Choy, “Over 1.1 eV Workfunction Tuning of Cesium Intercalated Metal Oxides for Functioning as Both Electron and Hole Transport Layers in Organic Optoelectronic Devices,” Adv. Funct. Mater. 24(46), 7348–7356 (2014). [CrossRef]
40. X. Li, F. Xie, S. Zhang, J. Hou, and W. C. H. Choy, “MoOx and V2Ox as hole and electron transport layers through functionalized intercalation in normal and inverted organic optoelectronic devices,” Light: Sci. Appl. 4(4), e273 (2015). [CrossRef]
41. P.J.L. Herve and L.K.J. Vandamme, “Empirical temperature dependence of the refractive index of semiconductors,” J. Appl. Phys. 77, 5476–5477 (1995). [CrossRef]
42. M. D. McCluskey, S. J. Jokela, and Defects in ZnO, “Defects in ZnO,” J. Appl. Phys. 106, 071101 (2009). [CrossRef]
43. E. Pál, V. Hornok, A. Oszkó, and I. Dékány, “Hydrothermal synthesis of prism-like and flower-like ZnO and indium-doped ZnO structures,” Colloids Surf., A 340(1-3), 1–9 (2009). [CrossRef]
44. A. B. Djurišić, Y. H. Leung, K. H. Tam, Y. F. Hsu, L. Ding, W. K. Ge, Y. C. Zhong, K. S. Wong, W. K. Chan, H. L. Tam, K. W. Cheah, W. M. Kwok, and D. L. Phillips, “Defect emissions in ZnO nanostructures,” Nanotechnology 18(9), 095702 (2007). [CrossRef]
45. G. Magesh, G. Bhoopathi, A. P. Arun, E. R. Kumar, C. Srinivas, and S. Sathiyaraj, “Study of structural, morphological, optical and biomedical properties of pH based ZnO nanostructures,” Superlattices Microstruct. 124, 41–51 (2018). [CrossRef]
